Laser diode based multiple-beam laser spot imaging system for characterization of vehicle dynamics

ABSTRACT

The invention is related to a laser diode based multiple beam laser spot imaging system for characterization of vehicle dynamics. A laser diode based, preferably VCSEL based laser imaging system is utilized to characterize the vehicle dynamics. One or more laser beams are directed to the road surface. A compact imaging system including an imaging matrix sensor such as a CCD or CMOS camera measures locations or separations of individual laser spots. Loading status of vehicles and vehicles&#39; pitch and roll angle can be characterized by analyzing the change of laser spot locations or separations.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application a continuation of U.S. patent application Ser. No.13/318,153, filed on Oct. 31, 2011, which is the U.S. National Phaseapplication, under 35 U.S.C. § 371 of International Application No.PCT/IB2010/051687, filed on Apr. 19, 2010, which claims the benefit ofEP Patent Application No. EP 09159068.7, filed on Apr. 29, 2009. Theseapplications are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The invention generally concerns measurements of vehicle dynamics suchas speed and roll angle. More specifically, the invention relates tooptical measurements of vehicle dynamics.

BACKGROUND OF THE INVENTION

It is known that laser self-mixing interference (SMI) can be used forspeed and distance measurements, see G. Giuliani, M. Norgia, S. Donati,T. Bosch “Laser Diode Self-mixing Technique for Sensing Applications” inJournal of Pure and Applied Optics, 6 (2002), page 283-294. It is alsoknown that vertical cavity surface emitting laser (VCSEL) withintegrated photodiode is particularly suitable for SMI sensingapplications. However, a general problem with speed measurements usingself-mixing sensors is that the orientation of the sensor with respectto the road directly influences the measurement. As the vehiclefrequently changes its orientation with respect to the road, e.g., dueto a declination of the vehicle body induced by centrifugal forces in acurve, measurement errors are induced. Thus, the absolute measurementaccuracy of a self-mixing ground speed sensor is compromised by vehicledynamics, namely, roll and pitch movement of vehicles.

OBJECT AND SUMMARY OF THE INVENTION

It is therefore an object of the invention to improve optical laserbased vehicle dynamics sensors. This object is solved by the subjectmatter of the independent claims. Advantageous embodiments andrefinements of the invention are defined in the dependent claims.

According to the invention, a laser diode based, preferably VCSEL basedlaser imaging system is utilized to characterize the vehicle dynamics.In the simplest case a single laser beam, preferably multiple laserbeams are directed to the reference surface, i.e. the road surface. Acompact imaging system including an imaging matrix sensor such as a CCDor CMOS camera measures locations, separations or distances ofindividual laser spots. Loading status of vehicles and vehicles' pitchand roll angle can be characterized by analyzing the change of laserspot locations or separations.

In combination with a laser self-mixing ground speed sensor, themultiple-beam laser spot imaging system according to the invention cansignificantly improve the absolute accuracy of vehicles' ground speedmeasurement. Pitch and roll angles derived from the laser imaging systemcan be implemented in a rotation matrix to correct the systematic errorof the ground speed measurement.

In combination with a laser self-mixing ground speed sensor, theinventive multiple-spot laser imaging system improves the reliability ofvehicles' ground speed measurement. The laser output power, focusquality of the beams, contaminations to sensor exit windows and roadsurface reflectance can be continuously analyzed by monitoring thecontrast ratio of the beam spots on the road. Abnormal changes indicatelaser failure, out-of-focus sensing beam, severe contaminations tosensor exit or entrance window or presence of extremely low reflectanceroads.

Since typical wavelength of near-infrared VCSEL (e.g of 0.86 μm) iswithin the spectral response range of conventional CCD or CMOS camera,preferably either a low cost CCD or CMOS camera is employed as thesensing component of the multi-spot laser imaging system.

In particular, an optical vehicle laser sensor system for detection ofvehicle dynamics parameters is provided, comprising

-   a laser device which generates at least one laser beam, so that the    laser beam generates a laser spot on a reference surface placed    opposite to the laser device,-   an imaging device comprising at least one matrix sensor with a lens    for imaging the laser spot onto the reference surface, the imaging    device being arranged so that the laser spot onto the reference    surface is visible within a field of view of the imaging device,    whereby the optical axis, resp. the viewing direction of the imaging    device and the direction of said laser beam are non-coincident,-   a detector for detecting a velocity of the optical vehicle laser    sensor system relatively to the reference surface from the signal of    the laser beam reflected or scattered back from the reference    surface,-   a data processing device for detecting the laser spot location    within the image data retrieved from the imaging device, and    calculating the orientation of said optical vehicle laser sensor    system from the spot location.

As the viewing direction or optical axis of the imaging device and thedirection of the laser beam are non-coincident, the laser spot locationwithin the image taken by the camera is dependent on both the distanceof the laser device to the reference surface and the polar angle of thelaser device with respect to the perpendicular of the reference surface.It is in this regard advantageous to emit the laser beam under an anglewith respect to the viewing direction. Compared to a laser beamspatially separated but parallel to the viewing direction, an angle ofthe beam to the viewing direction causes a greater displacement of thelaser spot in the image as a function of vertical displacement of thelaser device.

The sensitivity of the determination of the orientation can beconsiderably enhanced and ambigiuities in the measurement can beeliminated if more than one, in particular at least three spatiallyseparated laser beams are employed.

Thus, according to a preferred refinement of the invention, the laserdevice generates three spatially separated laser beams, so that thethree laser beams produce three laser spots on the reference surface,wherein two pairs of the spots are separated along two different lateraldirections along the reference surface.

In particular, lateral distances between the laser spots may bedetermined and the orientation of the optical vehicle laser sensorsystem with respect to the reference surface may be calculated on thebasis of the lateral distances.

The spatial separation of the laser spots along two non-coincidentdirections allow to measure the changes of mutual distances of the spotsalong these directions and thus provide information about a tilt inarbitrary direction.

In case of a road vehicle such as a car, a motor cycle, a truck or abus, the road surface may advantageously used as the reference surface.The orientation determined by the data processing device may include theroll angle and the pitch angle of a vehicle on which the sensor systemis mounted. The roll angle is the angle of rotation of the vehiclearound the forward or heading direction. The pitch angle is the angle ofrotation around an axis vertically to the driving direction and parallelto the reference surface, or road surface.

It is not necessary to calculate these angles in terms of degrees.Rather, numbers may be calculated which represent these angles.Accordingly, instead of the angles, parameters equivalent thereto may bedetermined by the data processing device. Preferably, all of theseangles, or equivalent parameters, respectively, are determined toprovide detailed information on the vehicle dynamics. Furthermore, alsothe slip angle (also referred to as the yaw angle) may be determined.This angle is the angle of rotation around an axis vertically to theroad surface, or reference surface, respectively. This angle can bederived from a comparison of the forward and lateral velocitiesdetermined from the measurement of the self-mixing oscillationfrequencies.

Similarly to the embodiment using a single laser beam, the variation ofthe distances of the laser spots can be advantageously enhanced, if atleast one of the laser beams impinges onto the reference surface underan angle with respect to the reference surface normal. It is thereforeadvantageous, if at least two of the three laser beams are emitted underdifferent angles so that these beams are non-parallel.

There are various detection principles for the detection of the velocitywith respect to the reference surface. For example, the detection may bebased on a time-of-flight measurement. Preferably, however, thedetection is based on measurement of self-mixing laser poweroscillations. If a part of the laser light scattered or reflected backalong the optical path into the cavity, the coherent superposition ofthe back-reflected light and the light generated in the cavity causesintensity oscillations. A particular accurate measurement employing thisprinciple can be performed using Doppler velocimetry.

For this purpose, the single laser beam or at least one of a multitudeof laser beams has a direction with a component along the velocitydirection. This can be easily fulfilled by using a beam whose directionincludes an angle to the perpendicular of the reference surface. TheDoppler effect then introduces a time varying phase shift to thereflected laser beam. If the velocity is constant, this phase shiftresults in periodically varying laser intensity. The frequency of thisoscillation is directly proportional to the velocity. Thus, according toa refinement of the invention, the detector for detecting a velocity ofthe optical vehicle laser sensor system relatively to the referencesurface comprises a detector for detecting a self-mixing laser intensityoscillation and circuitry for determining the frequency or period of theoscillation.

The laser intensity may be measured using a monitor photodiode as adetector. It is also feasible to measure variations in the voltageacross the laser cavity or the laser current, respectively.

The beams may be generated by splitting a laser beam. However, it ispreferred to use three laser diodes, one for each beam. Inter alia, thisis advantageous to separate components of movement using self-mixingDoppler velocimetry, since the self-mixing oscillations can bedetermined for each beam separately.

Further, a preferred type of laser diodes are vertical cavity surfaceemitting laser diodes (VCSELs). These types of laser diodes generallyproduce beams having better defined beam profiles compared to edgeemitting laser diodes. Furthermore, VCSELs can be easily produced aslaser diode arrays on a single chip without considerably increasing theproduction costs with respect to a single laser diode. Thus, accordingto a refinement of the invention, the laser device comprises a chip withthree VCSEL mesas thereon. As the inventive device employs multiplebeams, such an array of VCSEL laser diodes is particularly suitable forthe laser device.

Furthermore, it is advantageous to employ near infrared emitting laserdiodes, i.e. laser diodes emitting at a wavelength of at least 800Nanometer. Although typical near-infrared VCSEL laser light of awavelength around 0.86 μm is invisible to human eyes, it cannevertheless be easily detected by conventional CCD and CMOS cameras,whose spectral response range reaches up to 1 μm. When the beam isdirected onto the road surface, the individual laser spots can be easilyimaged with strong contrast even at presence of high ambient light.Thus, according to a refinement of the invention, the laser devicegenerates laser beams having wavelengths between 800 and 1000Nanometers.

In combination with the speed sensor, or detector for detecting avelocity, respectively, both the accuracy and reliability of vehicle'sground speed measurement can be greatly improved. Once vehicles' pitchand roll angles are known from the inventive multi-spot laser imagingsystem, systematic errors of the speed sensor induced by vehicledynamics can be corrected accordingly.

Thus, according to a refinement of the invention, the data processingdevice is set up to calculate a pitch angle, and a roll angle and tocorrect a velocity measured by the detector based on the pitch angle androll angle.

The slip angle may be determined by comparison of the forward andlateral velocity and can be corrected on the basis of the determinedroll and pitch angles.

If the laser beams include an angle, not only the absolute position ofthe spots within the acquired image but also their mutual distances varywith the distance of the laser device to the reference surface. In thiscase, the distance of the laser device to the reference surface can thusbe calculated by the data processing device from the separation ormutual distance of the spots. As the spot position changes as well, thedistance to the road surface may also be determined from the location ofa laser spot.

Furthermore, failure of laser diodes, contaminations to an exit windowof the sensor, out-of-focus sensing beam and presence of low reflectanceroad can be revealed by monitoring the changes of the contrast ratio ofindividual VCSEL focal spots.

The accuracy of the system can be further considerably enhanced if theoptical vehicle laser sensor system comprises a second or further laserdevice laterally offset to the first laser device. If there is aspecific forward direction, such as the forward driving direction of avehicle, it is in this regard furthermore advantageous if the laserdevices are spaced apart both along this forward direction andtransversally to the forward direction. It is preferred to provideseparate image sensors or cameras for both laser devices so that thecameras can be placed near the reference surface. The measurement of thedistance of the laser device to the reference surface is particularlysensitive. As in case of both a roll angle and a pitch angle, thedistances of the laser devices to the reference surface vary, theseangles can be calculated by the data processing device from the measureddistances.

Generally, the lasers are sufficiently intense to provide sufficientcontrast in the recorded images so as to unambiguously detect the laserspot locations. However, the contrast can be further improved bysuppressing the background light. According to one embodiment, apreferably narrow bandpass filter which transmits the laser light isarranged upstream to the matrix sensor to block background light.Preferably, the maximum transmission of the filter is chosen to be at ornear the laser wavelength

According to a further alternative or additional embodiment, the laserdiode is operated in a pulsed mode (e.g. square wave), temporallysynchronized imaging further improves the contrast ratio of the laserspots. In particular, a pulsed power supply for the laser device isprovided for pulsing the laser beams. The imaging device is synchronizedwith the pulsed power supply so that images are acquired during a pulseand between two pulses. To suppress the background signal, the dataprocessing unit can simply subtract the images.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows spectral response curves of a CCD-camera and a CMOS camera.

FIG. 2 shows an image of the laser spots on the road surface.

FIG. 3 illustrates an embodiment of an optical vehicle laser sensorsystem

FIG. 4 depicts a further embodiment with two spatially separated laserdevices,

FIG. 5 shows the orientation of a laser beam.

FIG. 6 shows a further embodiment of an optical vehicle laser sensorsystem.

DETAILED DESCRIPTION OF EMBODIMENTS

The optical vehicle laser sensor system for detection of vehicledynamics parameters according to the invention is based on a laserdevice which generates three spatially separated laser beams directedonto the road surface, so that three laterally separated laser spots onthe road surface are produced. An imaging device with a matrix sensorimages the laser spots. The speed of the vehicle is determined fromDoppler-induced self-mixing laser intensity oscillations. The lasersensor system further comprises a data processing device for calculatinglateral distances between the imaged laser spots, and determining theorientation of the optical vehicle laser sensor system with respect tothe road surface, or the vehicle's orientation with respect to the road,respectively.

A VCSEL emitting in the infrared spectral region between 800 and 1000nanometers wavelength is particularly preferred as laser diode. AlthoughVCSEL beams of self-mixing ground speed sensors are in this caseinvisible for human eyes, they can nevertheless be readily imaged withconventional CCD or CMOS camera, as can be seen in FIG. 1. The dottedline shows the spectral response of a CMOS sensor and the continuousline is the spectral response of a CCD sensor. As indicated in FIG. 1,typical wavelength of a near-infrared VCSEL (for example: 0.86 μm,indicated by the dashed vertical line) is within the spectral responseranges of both CCD and CMOS sensors.

Assuming as an exemplary embodiment that all VCSELs of the laser deviceare focused at the road surface with a numerical aperture of about 0.02,the radius of VCSEL focus at road surface is about 26 μm. An opticalpower of only 1 mW from a typical VCSEL can produce a power density atroad surface of 4.7 MW/m². In contrast, maximum irradiation of full sunis only 1 kW/m². Thus, even at presence of high ambient light,brightness of VCSEL focus spot is at least three orders of magnitudehigher than that of background. Therefore, the VCSEL focus spots can bevisualized with very high contrast even with low cost CCD or CMOScameras, which is verified in FIG. 2, showing a color inverted image ofthe three laser spots 10, 11, 12 on the road taken with a low-costmatrix-camera.

FIG. 3 shows a first embodiment of an optical vehicle laser sensorsystem 1. The laser device 3 is mounted on a vehicle at a distance Zabove the road surface 2. The laser device emits three laterallyseparated laser beams 30, 31, 32. The laser beams are emittednon-parallel so that an angle is included both between beams 30, 31 andbeams 31, 32. Due to these angles, not only the imaged spot positionsbut also their mutual distances vary if the laser device is tilted withrespect to the road surface 2 or displaced vertically thereto alongdirection Z.

Furthermore, as the laser beams hit the road surface 2 under an obliqueangle, Doppler induced phase shifts for a movement in lateral directionalong the road surface are induced into the reflected light so that thelaser intensity of the laser diodes can be evaluated to extractself-mixing oscillations and determine the vehicle velocity therefrom.

A camera 4 is placed nearby the laser beams 30, 31, 32 so that the laserspots on the road surface 2 lie within the camera's field of view 40.

If for example, the vehicle tilts about its main heading direction orforward direction 13, the distance AY between the spots of beams 31 and32 will change. The angle of rotation about this direction is referredto as the roll angle θ. On the other hand, a tilt of the vehicle bodyabout an axis 14 extending vertical to direction 13 and parallel to theroad surface 2 alters the position and mutual lateral distance of thespots of laser beams 30 and 31. The angle of rotation about this axis 14is referred to as the pitch angle.

If the distance of the laser device to the road surface 2 decreases, themutual distances between all spots will decrease as well, and viceversa. Thus, the distance to the road can be calculated from the mutualdistances ΔX and ΔY of the laser spots as well.

The configuration of a further embodiment of an optical vehicle lasersensor system is illustrated in FIG. 4. According to this embodiment, afirst laser device 3 and a second laser device 5 are employed, which arearranged laterally offset at two different positions on the vehicle.Separate cameras 4, 6 are provided for each laser device 3, 5.Specifically, the laser devices 3 and 5 are spaced apart both along theforward direction 13 by a distance b and transversally thereto alongaxis 14 by a distance a.

The VCSEL focus spot separations ΔX between laser beams 30, 31 and 50,51 are proportional to the height of the respective laser devices 3, 5relative to the road surface 2:

$\frac{\Delta\; X_{0}}{\Delta\; X^{\prime}} = \frac{Z_{0}}{Z_{0} + {\Delta\; Z}}$

where Z₀ and ΔX₀ denote the mounting height of VCSELs in a static,non-loaded vehicle and the corresponding VCSEL focus spot separations atthe road surface, respectively. Actual laser spot separations atpresence of vehicle dynamics are denoted as ΔX′₁ and ΔX′₂. Change ofheight of laser devices 3, 5, induced either by pitch/roll and/orloading is denoted as ΔZ₁, ΔZ₂. Considering a typical 4.5 m long vehiclewith a chassis height of 15 cm (Z₀), a pitch angle of 1 degree canproduce a change of height ΔZ of 4 cm, which corresponds to 26% relativechange in ΔX. Therefore, a low cost CCD or CMOS camera with less than20000 Pixel, such as, e.g., only 10 K pixel will be sufficient for manyapplications.

As shown in another embodiment in FIG. 6, change of vehicle height ΔZcan also be derived from the laser spot location which is captured by aCMOS or CCD camera 4.

The laser beam impinges to the road surface under an oblique angle withrespect to the road surface normal. This causes a shift of the locationof laser spot 12 in dependence of a height shift ΔZ, which can beexpressed by:

$\frac{\Delta\; Y}{\Delta\; Y_{0}} = \frac{\Delta\; Z}{Z_{0}}$

In this relation, ΔY₀ denotes the separation between a laser groundspeed sensor 3 and a camera system 4. ΔY indicates the distance betweenlaser spot 12 and the central optical axis 41 of camera 4. Forsimplicity, the laser beam of sensor 3 which is mounted in an unloaded,static vehicle is focused at road surface and crosses the centraloptical axis of camera 4. Accordingly, vehicle dynamics parameter canalready be obtained by monitoring a single laser beam. If the laser beamalso has a component in x-direction,

Of course, the monitoring of laser spot position shifts in addition oralternative to a relative measurement of their mutual distance can beapplied to a multi-beam device as shown in FIGS. 3 and 4 as well.Furthermore, a shift in position is even observed in case that laserbeam 30 and optical axis of the camera 4 are non-coincident butparallel. This is due to the fact that the magnification factor of thecamera depends on the distance.

With an arrangement using two laser devices 3, 5 as shown in FIG. 4, thevehicles' pitch (ψ) and roll (θ) angles can be derived from change ofVCSEL mounting height (ΔZ) at the different positions of the laserdevices:

$\theta = {{\frac{{\Delta\; Z_{1}} - {\Delta\; Z_{2}}}{a}\mspace{31mu}\psi} = \frac{{\Delta\; Z_{1}} - {\Delta\; Z_{2}}}{b}}$

Thus, instead of utilizing inertial or angular sensor to characterizevehicle dynamics, the multi-beam laser imaging system provides aneffective alternative to monitor vehicles' pitch/roll movement andloading conditions. Particularly, in combination with a laser groundspeed sensor, the accuracy and reliability of vehicle's ground speed andslip angle measurement can be significantly improved, as is elucidatedin more detail in the following. For the purpose to improve accuracy ofground speed and slip angle measurement, the data processing devicecalculates the pitch angle and roll angle, as explained above and thencorrects the velocities (i.e. the values of the velocity vector)measured by the detector based on the calculated pitch angle and rollangle.

The vehicle's ground speed or velocity vector V₀=(Vx, Vy, Vz) is derivedfrom the Doppler frequency vector (f₁, f₂, f₃), e.g., measured by aphotodiode which is integrated to each VCSEL. The frequencies f₁, f₂, f₃are the frequencies of the self-mixing oscillations of the respectivelaser intensities. The relation of the frequencies f₁, f₂, f₃ and thevelocities Vx, Vy, Vz (i.e. the cartesian components of the velocityvector) is given by:

$\begin{pmatrix}f_{1} \\f_{2} \\f_{3}\end{pmatrix} = {\frac{2}{\lambda}\begin{pmatrix}{\sin\;\theta_{1}\cos\;\varphi_{1}} & {\sin\;\theta_{1}\sin\;\varphi_{1}} & {\cos\;\theta_{1}} \\{\sin\;\theta_{2}\cos\;\varphi_{2}} & {\sin\;\theta_{2}\sin\;\varphi_{2}} & {\cos\;\theta_{2}} \\{\sin\;\theta_{3}\cos\;\varphi_{3}} & {\sin\;\theta_{3}\sin\;\varphi_{3}} & {\cos\;\theta_{3}}\end{pmatrix}\begin{pmatrix}V_{x} \\V_{y} \\V_{z}\end{pmatrix}}$

In this matrix equation, the angles θ₁, θ₂, θ₃, denote the polar anglesof the three laser beams measured with respect to the perpendicular ofthe road surface. The angles ϕ₁, ϕ₂, ϕ₃, denote the azimuthal angles ofthe beams measured with respect to direction 14 perpendicular to theforward direction 13. The orientation of these angles with respect tothe forward direction 13 and direction 14 is shown in FIG. 5 for one ofthe laser beams (i.e beam 30).

At presence of vehicle dynamics, the measured speed vector V=(V_(x),V_(y), V_(z)) can be corrected with a rotation matrix M_(R) in order toderive the true vehicle ground speed V₀=(V_(x0), V_(y0), V_(z0))according to the equation

V₀ = M_(R)⁻¹V, where  M_(R)  is  a  matrix: $M_{R} = \begin{pmatrix}{\cos\;\theta\;\cos\;\phi} & {{\sin\;\psi\;\sin\;\theta\;\cos\;\phi} + {\cos\;\psi\;\sin\;\phi}} & {{{- \cos}\;\psi\;\sin\;\theta\;\cos\;\phi} + {\sin\;\psi\;\sin\;\phi}} \\{{- \cos}\;\theta\;\sin\;\phi} & {{{- \sin}\;\psi\;\sin\;\theta\;\sin\;\phi} + {\cos\;\psi\;\cos\;\phi}} & {{\cos\;\psi\;\sin\;\theta\;\sin\;\phi} + {\sin\;\psi\;\cos\;\phi}} \\{\sin\;\theta} & {{- \cos}\;\theta\;\sin\;\psi} & {\cos\;\theta\;\cos\;\psi}\end{pmatrix}$

Accordingly, to obtain the corrected vector V₀, the measured vector ismultiplied with the inverse of the matrix M_(R). In the above equations,θ denotes the roll angle and Ψ the pitch angle. ϕ denotes the anglebetween the reference orientation of the laser device, or its forwarddirection, respectively, and vehicle's forward direction. This anglemay, e.g., occur due to mounting inaccuracies of the laser device.

The angle ϕ may be determined in a calibration procedure. In particular,if multiple beams are used and a determination of the transversal orlateral speed can be obtained from the self-mixing signals of therespective laser diodes, the angle ϕ can be obtained from the remaininglateral speed if the vehicle is moving straight ahead. In this case, theangle ϕ can be calculated according to the relationϕ=arctan(V_(x)/V_(y)), wherein V_(y) denotes the forward speed and V_(x)the transversal speed in a dynamic state without transversalacceleration.

Besides ground speed, the body slip angle of a vehicle is anothercritical parameter relevant to vehicle dynamics control. Therelationship between measured (β) and real (β₀) vehicle's body slipangle can be approximated by the data processing device according tofollowing equation:

$\beta = \frac{{\cos\;{\theta\beta}_{0}} + {\sin\;\theta\;\sin\;\psi} - {\cos\;\psi\;\sin\;{\theta\left( \frac{V_{z\; 0}}{V_{y\; 0}} \right)}}}{\cos\;\psi}$

Again, θ denotes the roll angle, Ψ denotes the pitch angle and ϕ denotesthe angle between the reference orientation of the laser device, or itsforward direction, respectively, and vehicle's forward direction. V_(z0)and V_(y0) denote the corrected vertical and forward velocities. Thesevelocities may be corrected according to the above matrix equation. Thebody slip angle is the angle between the vehicle's actual heading (orforward) direction and its longitudinal axis. This angle is measuredsimilarly to angle ϕ according to the relation β=arctan(V_(x)/V_(y)),wherein V_(y) denotes the forward speed and V_(x) the transversal speed.In difference to angle ϕ, the body slip angle typically occurs during atransversal acceleration, e.g. while driving a turn, while angle ϕoccurs due to a misalignment of the laser device and the vehicle'slongitudinal axis. Thus, according to a refinement of the invention, themeasured body slip angle, e.g. measured by comparison of the forward andlateral velocities is corrected using the above equation.

Once the pitch and roll angles are known from the multi-spot laserimaging system, systematic errors of SMI ground speed sensor can becorrected with the rotation matrix M_(R). Thus, the absolute measurementaccuracy of ground speed and slip angle can be greatly improved.

Besides accuracy improvement, the optical vehicle laser sensor systemcan improve the reliability of a ground speed sensor. Output power ofindividual VCSEL, focus quality of each sensing beam and reflectance ofroad surface are continuously analyzed by measuring the brightness orcontrast ratio of each VCSEL focal spots.

An abnormal reduction in contrast ratio may indicate VCSEL failure,out-of-focus sensing beam, severe contaminations to a sensor exit windowor presence of very low reflectance road surface. An early detection ofsuch events is particularly advantageous for an optical sensor (e.g. SMIground speed sensor) which can be used for vehicle stability control andis exposed directly to the harsh environment.

Without requiring conventional inertial or angular sensor, VCSEL basedmultiple-beam laser spot imaging system is able to measure vehicles'roll, pitch angle and loading status. The system can be used for vehicledynamics control, headlamp automatic leveling and advanced suspensionsystems. Particularly, in combination with a multi-beam self-mixingground speed sensor, both the accuracy and the reliability of vehicles'ground speed and slip angle measurements can be greatly improved.

The invention claimed is:
 1. An optical vehicle laser sensor system,comprising: a laser device arranged to generate at least one laser beam,the laser beam have a laser beam direction, wherein the laser beam isarranged to produce a laser spot on a reference surface, an imagingdevice, the imaging device comprising at least one matrix sensor,wherein the imaging device has an optical axis, wherein the optical axisof the imaging device and the laser beam direction are non-coincidentwith each other, a data processing device arranged to detect a locationof the laser spot, and to calculate an orientation of the opticalvehicle laser sensor system using the laser spot location.
 2. Theoptical vehicle laser sensor system of claim 1, wherein the laser deviceis arranged to generate three spatially separated laser beams, whereinthe three spatially separated laser beams generate three laser spots onthe reference surface, wherein at least two pairs of the laser spots areseparated along two different lateral directions along the referencesurface.
 3. The optical vehicle laser sensor system of claim 2, whereinthe data processing device is arranged to determine lateral distancesbetween the laser spots, wherein the data processing device is arrangeto determine the orientation of the optical vehicle laser sensor systemwith respect to the reference surface based on the lateral distances. 4.The optical vehicle laser sensor system of claim 3, wherein the dataprocessing device calculates a distance of the laser device to thereference surface based on the location of at least one of the laserspots.
 5. The optical vehicle laser sensor system of claim 3, whereinthe data processing device calculates a distance of the laser device tothe reference surface based on distances between the laser spots.
 6. Theoptical vehicle laser sensor system of claim 2, wherein each laser beamforms a reference angle with respect to the reference surface, whereinat least two of the three laser beams are directed to the referencesurface under different reference angles.
 7. The optical vehicle lasersensor system of claim 1 further comprising a detector circuit arrangedto detect a velocity of the optical vehicle laser sensor system relativeto the reference surface using a signal of the laser beam which isreflected from the reference surface.
 8. The optical vehicle lasersensor system of claim 7, wherein the detector circuit comprises: adetection circuit for detecting a self-mixing laser intensityoscillation; and circuitry for determining a frequency of theoscillation.
 9. The optical vehicle laser sensor system of claim 2,wherein the laser device comprises three laser diodes, each generatingone of the three laser beams.
 10. The optical vehicle laser sensorsystem of claim 1, wherein the laser device comprises three verticalcavity surface emitting laser diodes.
 11. The optical vehicle lasersensor system of claim 1, further comprising a second laser devicelaterally offset to the laser device, wherein the laser device andsecond laser device are spaced apart from each other both along aforward direction and transversally to the forward direction.
 12. Theoptical vehicle laser sensor system of claim 11, wherein the dataprocessing device is arranged to determine a first distance of the laserdevice to the reference surface, wherein the data processing device isarranged to determine a second distance of the further laser device tothe reference surface, wherein the data processing device calculatesfrom the first distance and the second distances a roll angle and apitch angle of the optical vehicle laser sensor system.
 13. The opticalvehicle laser sensor system of claim 1, wherein the data processingdevice is arranged to calculate at least one of a pitch angle and a rollangle of the optical vehicle laser sensor system, wherein the dataprocessing device is arranged to correct a velocity of the opticalvehicle laser sensor system based on the pitch angle and the roll angle.14. The optical vehicle laser sensor system of claim 13, wherein thevelocity of the optical vehicle laser sensor system is corrected bycalculatingV₀=M_(R) ⁻¹V, wherein V₀=(V_(x0), V_(y0), V_(z0)) denotes the correctedvelocity vector and V=(V_(x), V_(y), V_(z)) denotes the measuredvelocity vector, wherein M_(R) ⁻¹ is the inverse of matrix:$M_{R} = \begin{pmatrix}{\cos\;\theta\;\cos\;\phi} & {{\sin\;\psi\;\sin\;\theta\;\cos\;\phi} + {\cos\;\psi\;\sin\;\varphi}} & {{{- \cos}\;\psi\;\sin\;\theta\;\cos\;\phi} + {\sin\;\psi\;\sin\;\varphi}} \\{{- \cos}\;\theta\;\sin\;\phi} & {{{- \sin}\;\psi\;\sin\;\theta\;\cos\;\phi} + {\cos\;\psi\;\sin\;\varphi}} & {{\cos\;\psi\;\sin\;\theta\;\sin\;\phi} + {\sin\;\psi\;\cos\;\varphi}} \\{\sin\;\theta} & {{- \cos}\;\theta\;\sin\;\psi} & {\cos\;\theta\;\cos\;\psi}\end{pmatrix}$ wherein θ denotes the roll angle of the optical vehiclelaser sensor system, ψ the pitch angle of the optical vehicle lasersensor system, and Φ denotes an angle between a forward direction of thelaser device and a forward direction of the optical vehicle laser sensorsystem.
 15. The optical vehicle laser sensor system of claim 1, furthercomprising a pulsed power supply for the laser device, wherein theimaging device is synchronized with the pulsed power supply so thatimages are acquired during a pulse and between two pulses, wherein thedata processing unit subtracts the images acquired during a pulse andbetween two pulses.
 16. The optical vehicle laser sensor system of claim1, wherein the at least one matrix sensor comprises one of acharge-coupled device and a complementary metal oxide semiconductorsensor.
 17. The optical vehicle laser sensor system of claim 2, whereinthe orientation of the optical vehicle laser sensor system includes apitch angle of the optical vehicle laser sensor system and a roll angleof the optical vehicle laser sensor system, wherein the imaging deviceis configured to image the three laser spots on the reference surface,wherein the data processing device is arranged to calculate anorientation based on the three imaged laser spots, and to correct avelocity measured by the detector based on the orientation of theoptical vehicle laser sensor system.
 18. The optical vehicle lasersensor system of claim 7, wherein the velocity includes a forwardvelocity of the optical vehicle laser sensor system relative to thereference surface and a lateral velocity of the optical vehicle lasersensor system relative to the reference surface, wherein the detector isconfigured to detect the forward velocity of the laser system and thelateral velocity of the laser system from the signal of the laser beamreflected back from the reference surface.
 19. The optical vehicle lasersensor system of claim 18, wherein the orientation of the opticalvehicle laser sensor system includes a yaw angle of the optical vehiclelaser sensor system, wherein the data processing device is arranged tocalculate the yaw angle of the optical vehicle laser sensor system fromthe forward velocity of the laser system and the lateral velocity of thelaser system.
 20. An optical vehicle laser sensor system, comprising: alaser device arranged to be mounted on a vehicle, wherein the laserdevice is disposed above a road surface, wherein the laser device isarranged to generate at least three laser beams, wherein the laserdevice is arranged to produce three laser spots on the road surface,wherein at least two of the three laser beams are non-parallel with eachother, wherein at least one of the three laser beams impinges obliquelyon the road surface; an imaging device, comprising at least one matrixsensor; and a data processing device arranged to calculate values ofvehicle dynamics parameters of the vehicle on which the laser device ismounted based on a location of each laser spot, wherein the vehicledynamics parameters are dependent on a pitch angle of the vehicle and aroll angle of the vehicle.
 21. The optical vehicle laser sensor systemof claim 20, further comprising a detector, the detector arranged todetect a velocity of the vehicle relative to the road surface from asignal of at least one of the laser beams reflected from the roadsurface, wherein the data processing device is also arranged to correctthe velocity detected by the detector based on the vehicle dynamicsparameters, wherein the velocity comprises: a forward velocity of thevehicle; and a lateral velocity of the vehicle, wherein the detector isarranged to detect the forward velocity of the vehicle relative to theroad surface and the lateral velocity of the vehicle relative to theroad surface based on at least one of the laser beams reflected from theroad surface.
 22. The optical vehicle laser sensor system of claim 21,wherein the vehicle dynamics parameters comprise a yaw angle of thevehicle, wherein the data processing device is also arranged tocalculate the yaw angle from the forward velocity of the vehicle and thelateral velocity of the vehicle.